texts: What is left of a universe where no matter and energy exist and is there a term for this?

texts:
In the final energy state of the universe, there exists a hypothesis that all matter will have fallen into black holes and neutron stars. If, through Hawking radiation, after all matter is consumed and black holes eventually do dissipate, what is left of our Universe?

I may be describing Heat Death, but I'm not 100% sure. However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.

In the event I'm wrong and there exists no matter and no energy, would the term Universe still be applicable here?


texts: Why don't small stars end up as a black holes?

texts: I have recently done some research into black holes, and realized that big stars form black holes, whilst smaller ones don't. Is this because the gravity isn't strong enough for it to fall in on itself, or something else?


texts: How to build a powerful home made telescope

texts:
Simple, I fell in love with the sky when my parents were constructing our new house I was a kid then and one day I ventured out on the terrace, to my amazement it was a full moon, ever since I've felt spiritually connected with the Universe. Its been around 20 years since that day, out of my busy schedule I looked at the stars yesterday, I found love again! Wanted to buy the best telescope out there, but its out of my pocket.

Want to build a home made powerful telescope? Any help fellow sky lovers and star chasers?


texts:
I just saw articles which say the Harvest moon will be visible in the US.

Apart from the US, in which other country can people see it? Can I see it from India?


texts: Why can't moon light (reflected sun light) turn the sky blue?

texts:
Does turning the colour of the sky blue need more luminous light? Does it depend on luminosity or some other factors are also responsible for this phenomenon? Why can't the moon light turn the sky blue even a little bit (at least the area near the disc).

Thanks


texts: Did the Neptune and Pluto trajectories ever cross or was it a mere reclassification? Do studies say that I can predict an intersection between 2 trajectories  or movements based on data now? Do I have to consider at least 3 solid or larger space bodies moving in a way like the 3-body problem?


texts: Does the twin paradox work in an almost empty universe?

texts:
Background
The twin paradox is the popular thought experiment involving twins, one of whom makes a journey to a nearby star in a high-speed rocket (travelling at a velocity close to the speed of light) and when he reaches the star, changes direction and returns home at the same speed.  When he gets home, he  finds that time has slowed down during his journey and that his twin who has remained on Earth has aged more.  (The paradox being that because both twins have travelled at the same speed with respect to each other, why has one aged faster than the other?)
My understanding is that the solution is as follows. When he arrives at the star and changes direction, the travelling twin switches from a reference frame travelling away from Earth at a velocity near the speed of light to another reference frame travelling towards  Earth with a velocity of the same magnitude but in the opposite direction. In contrast the non-travelling twin stays in the same reference  frame. It is often pointed out that the travelling twin experiences an acceleration when he switches from one reference frame to another, whereas the non-travelling twin does not experience an acceleration. (This acceleration could be a Dirac delta function i.e. an instantaneous switch of direction)
Questions
My first question is what is the travelling twin’s acceleration with respect to? Since in special relativity there are no absolute reference frames. I assume it is the Earth??
Secondly, and this is the question which is intriguing me the most, suppose hypothetically there were only two objects in the Universe namely the two twins. If, in this hypothetical universe, one twin travelled on a rocket for a distance of say ten light years and then returned back to meet the original twin. Would the travelling twin’s clock  still run slow with respect to the other twin?  since there would  be no distant background stars/galaxies in this hypothetical universe  to measure the acceleration against? Is it just as valid to say that the “stationary twin” is the one in motion?
Additional comment
In a various replies to this question, it has been mentioned that the moving twin  feels an acceleration and the stationary one does not, but in this hypothetical Universe containing only the two twins I cannot see how this can be the case


texts: What practical considerations are there for amateur observations of transiting exoplanets?

texts:
Obviously, I am not referring to actual viewing of the exoplanets themselves, but detecting their effects on the brightness of the light emitted from the parent star (as in the diagram below from The Institute of Astronomy, University of Hawaii).

I would imagine that a good quality telescope, would be about to detect and record the effects of the exoplanet transiting the parent star.

What practical considerations in terms of equipment, software etc. would be required for 'backyard' amateur observations of the effects of transiting exoplanets?

If anyone has actually tried this, what has been your experience?


texts: Tools for simulation of theoretical astronomical problems?

texts: I tried looking around for open source tools that would allow me to run simulations and build on top of the code with my own need(mainly mathematical simulations, but 3D visuals would be a bonus.), unfortunately I haven't really found many open source projects, so I think maybe this tools are somehow available on websites like NASA or other Foundations for astronomical research. Where would one go to find the actual professional software for astronomy research and simulations ?


texts: Is there a public record of planetary disks apart from ours?

texts:
Our planetary disk is not aligned with the rest of the galaxy.  Are there publicly available records of the orientations of any other planetary disks?
Especially interested in nearby stars - but I'd expect that to be easier for us to measure?


texts: What is the difference between asteroids, comets and meteors?

texts:
Some celestial objects seem to be referred to as asteroids, some as comets, and some as meteors or meteorites.

What is the distinction between all of these different objects? Are any of them the same?


texts: What are the advantages of refractor telescopes over reflector telescopes?

texts: I've been thinking about getting a new telescope. From what I've read online, reflector telescopes are newer and better then refractor telescopes. Is there any reason to get a refractor telescope instead of a reflector?


texts: Is there a clear-cut difference between rocky and gaseous planets?

texts:
A gas giant planet is large enough that it retains a lot of hydrogen and helium. A rocky planet is one with a solid surface. That's the rule of thumb to distinguish between the rocky and gaseous planets.

But gaseous planets can have a solid core and rocky planets can have a very thick atmosphere. So is there a clear-cut difference between the two categories of planets (perhaps linked to different formation histories), or is there a continuum of planets that goes from more rocky to more gaseous? If it's a continuum, where is the limit set between the two types?


texts: Calculating azimuth from equatorial coordinates

texts:
I try to get the azimuth of an object from its equatorial coordinates using this formula:

$a = \arctan2(\sin(θ - α), \sin φ * \cos(θ - α) - \cos φ * \tan δ)$

Where
φ = geographic latitude of the observer (here: 0°)
θ = sidereal time (here: 0°)
δ = declination
α = right ascension

I made two JavaScript functions to implement this calculation:

  obliq = deg2rad(23.44);  // obliquity of ecliptic
  lat2  = deg2rad(0);  // observer's latitude
  lmst  = deg2rad(0);  // siderial time

  function equatorial(lat, lon) {
    // returns equatorial from ecliptic coordinates
    dec = Math.asin( Math.cos(obliq) * Math.sin(lat) + Math.sin(obliq) *
          Math.cos(lat) * Math.sin(lon));
    ra  = Math.atan2(Math.cos(obliq) * Math.sin(lon) - Math.sin(obliq) * Math.tan(lat),
          Math.cos(lon));
    ra += 2 * Math.PI * (ra < 0);
    return [dec, ra];
  }

  function horizontal(lat, lon) {
    // returns horizontal from ecliptic coordinates
    coords = equatorial(lat, lon);
    dec = coords[0];  // δ
    ra  = coords[1];  // α
    alt = Math.asin(Math.sin(lat2) * Math.sin(dec) + Math.cos(lat2) *
          Math.cos(dec) * Math.cos(lmst - ra));
    azm = Math.atan2(Math.sin(lmst - ra), Math.sin(lat2) * Math.cos(lmst - ra) -
          Math.cos(lat2) * Math.tan(dec));
    azm += 2 * Math.PI * (azm < 0);
    return [alt, azm];
  }

I cannot see any error, but I get strange results for azimuth (a) as can be seen in this table (the other values seem correct):

   λ       δ         α         h         a
    0    0.0000    0.0000   90.0000    0.0000
   15    5.9094   13.8115   75.0000  246.5600
   30   11.4723   27.9104   60.0000  246.5600
   45   16.3366   42.5357   45.0000  246.5600
   60   20.1510   57.8186   30.0000  246.5600
   75   22.5962   73.7196   15.0000  246.5600
   90   23.4400   90.0000    0.0000  246.5600
  105   22.5962  106.2804  -15.0000  246.5600
  120   20.1510  122.1814  -30.0000  246.5600
  135   16.3366  137.4643  -45.0000  246.5600
  150   11.4723  152.0896  -60.0000  246.5600
  165    5.9094  166.1885  -75.0000  246.5600
  180    0.0000  180.0000  -90.0000  248.3079
  195   -5.9094  193.8115  -75.0000   66.5600
  210  -11.4723  207.9104  -60.0000   66.5600
  225  -16.3366  222.5357  -45.0000   66.5600
  240  -20.1510  237.8186  -30.0000   66.5600
  255  -22.5962  253.7196  -15.0000   66.5600
  270  -23.4400  270.0000   -0.0000   66.5600
  285  -22.5962  286.2804   15.0000   66.5600
  300  -20.1510  302.1814   30.0000   66.5600
  315  -16.3366  317.4643   45.0000   66.5600
  330  -11.4723  332.0896   60.0000   66.5600
  345   -5.9094  346.1885   75.0000   66.5600
  360   -0.0000  360.0000   90.0000   68.3079

Does anyone see the error? Thank you.


texts: Formation of Cosmic Microwave Background

texts: It is said that the cosmic microwave radiation (CMB) was formed when the universe was 379,000 years old. How is this calculated?


texts: Could a super massive black hole actually be primordial black hole in disguise?

texts: If primordial black hole do exists today could it be responsible for galaxies maybe even quasars, I mean these ancient monsters gobble up all materials in its wake and grow bigger and eventually evolves into say milky way galaxy etc. What are the odds of such events actually happened?


texts:
Two Questions:

1) Are rogue planets in motion, i.e. are they just wandering freely in space or stationary?

1) Do rogue planets have defined path or a one which can be anticipated? I understand that they cannot be at absolute rest as they are moving along with the galaxy.

2) Do rogue planets rotate around their axes?

If they are in motion, and they outnumber the stars in our galaxy, why our solar system never had a guest?


texts: Direction of Earth’s poles relative to the Sun’s movement around the Galaxy

texts:
The plane of the solar system is at 63 degrees to the plane of the galaxy, then either the North or South Pole of Earth would be on the ‘leading edge,’ i.e. pointing roughly in the direction of movement around the galaxy (+/-23.5 degrees depending on the direction of the tilt of the earth at the time). Would there be any effect on the atmosphere at the front of this direction of travel caused by additional collision / friction with space dust or solar wind? For example, if the South Pole was on the leading edge, could something like the ozone hole be exacerbated by interstellar particles, or could the auroras be more spectacular at the pole that points forwards? If so, is there evidence of seasonal variation in these effects as the tilt of the earth increases or decreases that pole’s angle towards the direction of the sun’s movement along the galactic plane?

Also, would the plane of the planets be shifted slightly to the rear of the midline of the sun because the sun is, in effect, dragging them behind it?

Finally, is there any evidence of increase or decrease in the above postulated effect depending on whether the solar system is above or below the galactic plane during its 30 million year oscillation? E.g. if there were more interstellar dust while passing through the mid-plane, would that leave a 30 million year signature in polar ice-core samples?


texts: Why does the axial tilt & the rotation speed of Saturn & Jupiter vary so much?

texts:
I have seen Jupiter and its 4 largest moons in a telescope, several times. The moons are always aligned in a line. This is fine. Very interesting. But.. in case of Saturn, not only that we can see its ring, moons are not aligned in a line. I was wondering why is it so and I came to a conclusion that Saturns axial tilt must be bigger than Jupiters. I have visited english wikipedia and found out that it is true - and that the difference is about 23°.

Why is it so?

I am not a physicist. I do not even study astrophysics, but I wonder why the difference is so big. I came to another conclusion that the rotation speed may play a huge role here. The faster an object is spinning, the smaller axial tilt. And according to wikipedia, there is a difference and it seems that it fits in my theory.

So. I did my research, but now I am not sure if that´s all and if it´s right. I mean, maybe there is another factor that I forgot to count-in. I know that, according to scientists, Jupiter formed before Saturn. Does it mean that the difference in formation answer my question to why the axial tilt & the rotation speed vary so much? Or is it because of the distance to Sun?


texts: Is there a recent update on the mysterious M82 radio source reported in 2010?

texts:
The Wikipedia page on the "Cigar Galaxy" (M82) has the following summary about an "unknown object":

  In April 2010, radio astronomers working at the Jodrell Bank
  Observatory of the University of Manchester in the UK reported an
  object in M82 that had started sending out radio waves, and whose
  emission did not look like anything seen anywhere in the universe
  before.

A subsequent article in New Scientist dated 13 December 2010 offers the following update:

  The object is still a puzzle, says co-discoverer Tom Muxlow. “It was
  still there the last time we looked, so its lifetime is now well over
  a year,” he says. “We are continuing to monitor this object.”

I can find a number of other references to this unknown object, but nothing more recent than 2010. The articles suggest it could perhaps be a new type of microquasar, since it shares some similarities; however, it doesn't have the high X-ray output that known microquasars exhibit.

Has the mystery been solved, or is this still an area of active research?


texts: How would the solar system look in a Geocentric model?

texts: If I fix the Earth as the center of the solar system, what would this look like? Are there any good images? How complex is this system?


texts: Do gravitational waves really travel in "nothing"?

texts:
How does a Gravitational wave create ripple in the fabric of space, if space itself is an empty and it is made-up of nothing. Through out my academic life in school, I was told that space is made-up of nothing.

If so, then in which medium waves travel?

If they do indeed travel in "nothing", how is that possible?


texts: What is gravity?  I want to know more than it being simply the "mysterious force" that attracts things to earth.  Is it a particle, a wave, or something else entirely?


texts: Why, in the Solar System, all the mass seems to be concentrated at the centre?

texts: Is this a consequence of planet formation in accretion disks ?


texts: Why is there "modulus" word in the "distance modulus" term?

texts:
The difference of apparent (m) and absolute (M) magnitudes of an object is called "distance modulus", which is related to the distance $d$ to the object in parsecs with equation

$$m - M = 5 \log \Big(\frac{d}{10 \ \textrm{pc}}\Big) \tag{1}.$$

I'm trying to understand why $m - M$ is called "distance modulus". The word "distance" makes sense, because there is a distance d term in Eq. (1). But what is "modulus" word doing there? In math, modulus means the remainder after division of two numbers. For example, modulus of 5 and 3 is 2.

Another meaning of "modulus" is length of a vector. I don't see how either of these two meanings can be relevant to Eq. 1.


texts: Star density in- versus out- of the arms of the galactic pattern?

texts:
What is the density difference in our galaxy (or in a typical spiral),
at A, B, C versus at a, b, c?

Consider the yellow track.
Do astronomers have a graph like this...

...which shows the shape of the density through and between the arms?
In short,
Do we roughly know the density between-arms versus in-arms ... is it 10%, 50% or 90%?
(Perhaps at the "corotation radius", which is about where we are.)

Further issues arising:
= I understand that one of the reasons the arms "appear real" is just that there are more bright young stars forming in the arms (due to the higher pressure). Perhaps the actual density difference is very low?
= Perhaps the density variation is dependent on the type of star, or other material - what about things like gasses?
= Let's say the percentage I am asking about is X% - at our radius. Is it X% at all radii?  How does the "strengthiness" percentage vary with radius? This is not immediately intuitive from looking at images.
(original galaxy arm pattern diagram courtesy wikipedia)


texts:
In Time domain astronomy and fastest eclipsing binary ZTF J1539+5027 (+20 mag, 6.91 minutes) How to measure it's minimum brightness? I've written

A logarithmic magnitude scale might tend to show just how deep but noting that the minimum is dimmer than -21.5 mag and probably lasts only a tiny fraction of the orbital period (i.e. a few seconds) makes a good measurement of the minimum quite a challenge.
It seems to me like this might be a situation where a photomultiplier tube and photon counting might be competitive with photometry based on CCD imaging and clever readout schemes.

This has lead me to wonder more broadly:
Question: Are astronomers still using photomultiplier tubes for optical photometry?
These include position-sensitive PMTs but excludes other electron multiplication devices like channel plates and image intensifiers. I'm asking about vacuum tubes with one photocathode, a series of dynodes, and an anode where the multiplied photocurrent can be read out either as a DC average level for higher fluxes (possibly using lock-in techniques) or shaped pulse counting for low flux rates.


texts: How bright would Sagittarius A* be from Earth if it became an Active Galactic Nucleus?

texts:
Essentially I'd like to know how bright our central black hole could appear from earth in relation to full moon brightness. Assuming maximum energy output (like when it's gobbling up an entire globular cluster). I've read that the light from an AGN could actually support photosynthesis on planets within 1100 light years, so I'm curious if it would have any affect on Earth life 25000 light years away.

https://aasnova.org/2019/05/29/do-active-galactic-nuclei-help-or-hurt-life/


texts: Is it possible for a solar flare in one star to influence the chances of a solar flare in another star?

texts: Can a solar flare reach far enough to travel from one ordinary star to another?  If so, how can that solar flare affect the following star?


texts: Are the planets Trappist-1 in the solar system?

texts:
I'm an Astronomy noob, but I'm interested in learning how stars and planets work. Everything started with the Trappist-1. I read about it, and it's fascinating. However, it's impossible for me to localize it with my current state of knowledge. Therefore, I would like to ask some general questions here.

First of all, during high school, I have learned that we are in the solar system and there are some planets orbiting the sun. Easy to remember and imagine, but there are stars, planets, constellations, etc. which I can't manage to relate to this basic system that I learned about.

And what confused me a lot are these two pictures:

and

And this definition of an exoplanet:

  An exoplanet or extrasolar planet[...]

From the pictures, I can see that Trappist is in fact in the solar system. Is this right? And if this is right, why in the world did it take us so many years to find it? We have one Voyager that is almost out from the solar system. Couldn't it find it before? And is it possible to find more planets in our solar system? Is it because Voyager went only in the x-y direction, and not "perpendicular" to the orbits?

I'm sorry for asking these stupid questions, but I simply can't build new knowledge on my current low-level understanding of the solar system. This is my current knowledge:

Can you please help me to answer the above questions?


texts: Is it possible that the Sun and all the nearby stars formed from the same nebula?

texts: I was looking at a map of our local stellar neighborhood, and it occured to me, the stars are really close, if one compares them to the size of some nebulae. So can it be, that the Sun, Alpha Centauri system, and all the other stars in the neighborhood actually formed from the same nebula? Have we discovered any information about this?


texts: What's going on in this photo of Halley's comet's tail?

texts:
When researching for this answer I was looking at Space.com's 2011 article Photos of Halley's Comet Through History

I saw the image below, and read the caption.

  Halley's Comet in 1910

  NASA/JPL

  Halley's Comet as photographed May 13, 1910, by a wide-angle camera at Lowell Observatory, Flagstaff, Ariz., during the comet's last appearance. A streak across the comet near the coma is a meteor trail, and not a scratch on the negative. Streaks at the bottom right are the city lights of Flagstaff Bright spot above the city lights is the planet Venus.

I know that 109 years ago astrophotography wasn't what is is today, but the combination of the overexposed venus, the coincident meteor trails, the non-uniform star trails blending in with the comet's tail breakup along with the unlikely looking "smoke trail" from the upper side of the comet's tail make this photo look quite unusual.

Question: What's going on with that second tail thing above the main tail? The shape looks so unnatural, it seems to have a parallel extent rather than radiate from the coma. What caused this?

"bonus points" for identifying the photo in some catalog, or in a publication.


texts: Would we be able to feel the acceleration of a planet with a highly eccentric orbit?

texts:
Even though our Earth moves on average 67,000 miles per hour around the Sun, we cannot feel its movement since its speed is fairly constant. More precisely, its change in speed is very low and our bodies cannot detect that low of acceleration.

What if instead a hypothetical human observer were placed on a planet with a very eccentric orbit, say 0.99? Would that human be able to feel the planet's acceleration as it approached its host star, and as it receded from its host star? And if a human observer would not be able to feel any acceleration even at e=0.99, what is the minimum eccentricity, if any, that a planet must have in order for a hypothetical human observer to feel the effects of its acceleration and deceleration?


texts: What does "O I" mean in the context of detected molecules?

texts:
I've recently been analysing the Exoplanet Database, and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I2O), so I'm at a loss as to what it could be, especially since I'm very new to this field.

Any help is very much appreciated!


texts: Why is it always planets orbiting stars?

texts:
In our solar system, there are 8 planets orbiting a star, the Sun.
And I understand that there are about 500 confirmed solar systems out there.

But why is it always planets orbiting stars? Why can't it be several stars orbiting a planet, or a star orbiting a star? Why is a star by definition stationary as opposed to planets which are moving.

Of course, in that case, it wouldn't make much sense to call it a solar system, but still.


texts: What kind of collision of celestial bodies would be visible to the naked eye?

texts:
When asteroids crash into the Moon they give of huge amounts of energy resulting in a "bright" flash, as can be seen in this video. At the end of the video its also says that anyone looking at the Moon at that moment would have been able to observe it without a telescope.

But what kind of collision would be required to be able to be seen from Earth when it would involve more distant bodies? Would one of Jupiter's retrograde moons colliding with one of its prograde moons be visible?

Edit:
A real example of a high energy collision would be the collisions of the fragments of comet Shoemaker-Levy-9 with Jupiter. However according to this video from ESA it happened on the opposite side of Jupiter, so not in our view.


texts: How to convert from Cartesian J2000 coordinates to Cartesian Galactic coordinates?

texts:
I'm using a star database (the HYG database) which has xyz positions and velocities for the stars. This has +X towards the vernal equinox, +Z towards north celestial pole, +Y towards Right Ascension 6 hours and it's based on J2000 and centered on the sun.

I'd like to convert these to a Cartesian Galactic coordinate system centered on the sun, with +X pointing to its center and +Z angled so that it shoots out of the galactic plane on the north side.

This should just be a simple rotation, but after a lot of searching I can't seem to find the rotation to use, nor some way of converting it through a third system.
Does anyone know how to obtain the correct rotation matrix here? (or some other method of rotation)


texts:
Wikipedia tells us:

  In physical cosmology, cosmic inflation, cosmological inflation, or just inflation is the exponential expansion of space in the early universe. The inflationary epoch lasted from 10-36 seconds after the Big Bang to sometime between 10-33 and 10-32 seconds. Following the inflationary period, the Universe continues to expand, but at a less accelerated rate.

I've no idea about astronomy, I just wondered whether somebody ever published a theory about cosmological deflation, which would make the universe shrink in a fraction of a second.

In a SFI novel you could travel by locally causing a deflation of space in the direction of your spaceship; the unshrunken space of a lightyear would (only locally in front of the ship) measure 1000 km for the fraction of the second. Or maybe a technically very advanced civilization could keep its solar system inflated by the time of a deflation of the whole universe and thus survive, which leads to the interesting question, whether this solar system could reenter the space of the next universe after its new cosmolocical inflation and if yes, does somebody look out for stellar systems at places in space where you wouldn't expect them?

However: cosmological deflation?


texts: I was just wondering what are the chances that there is a small object (say less than 1 km but more than few meters) that orbits the Earth but has remained undetected by us? Are we actually scanning the space around the Earth continuously for orbiting bodies?


texts:
Two questions:

I read online that all objects emit the majority of their thermal radiation in the radio, and that this emission increases with temperature. Does this mean that all glowing bodies emit the majority of their radiation in the radio, regardless of their temperature?
Wien’s law is used to find the peak wavelength of a black body. Does this mean that there is zero emission at any wavelength shorter than this peak wavelength?

Thank you in advance for your help.


texts: What is the fifth moon-like object I saw around jupiter through my telescope?

texts:
Tonight while observing Jupiter I was able to see a five (what looked like) moons. I have observed Jupiter often before and only been able to make out the four galilean moons. It's my understanding that through my 8-inch scope it would be impossible for me to see any others. Switching to a widefield lens I was able to see Jupiter along with some nearby stars however the 5 moon-like points around it all still appeared... well, moon-like in comparison to the bright stars.

I observed Jupiter from about 2016-04-15 05:20 to 05:40 UTC and all five of the moon-like objects were present the whole time. What was I probably looking at?


texts: Today at about 2 am I have spotted a light very similar (perceived speed and apparent brightness, motion due East) to ISS. At very first I thought to have spotted the station but immediately I doubted about as I feel that the observation time is too far from sunrise. ISS is indeed ruled out as there is not email from the spot the station service in my inbox. Can satellites orbiting at higher orbits be see even in deep night?  I feel that answers is yes based on simple geometry. However what could reflect so much light? I think there are not objects with reflecting surface so big to be compared to those of ISS...


texts: A curious relationship between lunar periods and the solar year

texts:
Earlier today I was exploring the astronomy features on WolframAlpha, when I stumbled upon an intriguing numerical relationship involving the values for the lunar synodic and sidereal months. Using the values

$$P_{syn}=1\,\text{synodic month}=29.530588\,\text{days},~~P_{sid}=1\,\text{sidereal month}=27.321661\,\text{days},$$

I noticed that this algebraic combination is very close to one year:

$$\frac{1}{P_{sid}^{-1}-P_{syn}^{-1}}=365.256396\,\text{days}\\
\frac{1}{P_{sid}^{-1}-P_{syn}^{-1}}-1\,\text{tropical year}=0.0142053\,\text{days}\approx20\,\text{minutes}.$$

I really doubt this this is a coincidence. Is there a simple way to understand what makes this approximate equality hold?


texts:
Why are all quasars so far away?

If the universe is homogeneous, we could expect to have a homogeneous distribution of quasars, but all of then seem to be far away from Earth. Why is that?


texts: I am high school student, I  have doubt if I have to locate a planet or any star in space, how do I locate it, because we have no perfect coordinate system, I have read in some books, that time is define as the one of the coordinate, in coordinate system, can we define something only by time as coordinate.


texts: Inflation cosmology: slow-roll inflation versus tunneling between two vacua

texts:
In slow-roll inflation models, the early inflation of the universe is driven by the flat non-zero part of the inflaton potential, and it ends as the ball rolls down the cliff and the potential energy released leads then to the formation of matter.

However, as I have heard such slow-roll models are no longer deemed realistic today in inflationary cosmology, instead other models where the end of the inflation is explained by quantum tunneling of the universe from a state with higher vacuum energy density to another state with lower vacuum energy density are considered for example.

Why (from a physical point of view) are slow-roll inflation models no longer considered to be realistic, what are their disadvantages? What can models that explain (the end of) the early inflation by a tunneling transition between to vacua explain or describe, that slow roll models can not do?


texts: When does oscillation of interplanetary plasma occurs?

texts:
When does oscillation of interplanetary plasma occur?

When and what are the reasons of the excitation of this plasma that makes it oscillate? Does it oscillate when is not excited?

Can extremely low radio frequencies pass through it when it is not oscillating?


texts:
Many papers mention emission measure, and some of them give the expression of EM, BUT there is no clear definition of EM.

My question is what EM is on earth? Its unit is supposed to be cm-3.

How to calculate EM? here(4B5) gives an expression, but it can not be derived from a light curve directly.

Given an X-ray light curve(not from the Sun), probably I need to devide the light curve into many bins and do spectral fitting.

Then what should I do? Rererence papers with clear definition and expression are welcome.


texts: What causes the duration of moon phases (the time the moon is visible above the horizon) to vary for an observer in the northern hemisphere?

texts: Why does each phase of the moon stay up a different number of hours per phase, so that, for example, January’s full moon was up twice as long as the new moon? Plus every month this bulge shifts and at some point the opposite phase is doubled. Three June full moon nights takes six March 1st quarter nights. Why does this happen?


texts: Astrophysical "unitised" version of the Gravitational constant

texts:
This question relates formalising the gravitational constant, $G$, (which in S.I. units is usually quoted as $\sim6.67\times10^{-11}\,\mathrm{m}^{3}\,\mathrm{kg}^{-1}\,\mathrm{s}^{-2}$) in units which are more appropriate to astrophysics, such as in the field of galaxy dynamics.

What would $G$ be in more appropriate units to astrophysics?


texts: Fate of the Spiral Arms of the Milky Way and Andromeda

texts:
In a few billion years, the Milky Way and Andromeda will collide. Chances are, there won't be many spectacular stellar collisions, and there's a pretty good chance that the Solar system (and most other stellar systems) won't be flung out into deep space, or be torn apart by incoming stars. But what about on a larger scale?

The product of the collision - dubbed "Milkomeda" - will be an elliptical galaxy, unlike the Milky Way and Andromeda, which are both spiral galaxies. My question is this:

How will the structure of the spiral arms be effectively destroyed by the collision? What mechanisms will cause them to coalesce, and what structures will take their place?


texts: Variation in g acceleration due to gravity

texts:
I want to find a routine which will enable me to calculate the variations in g
as a result of the motion of sun and moon. I need to have the date as a variable.
I realise that Dennis Milbert's earth tide routine must contain this information, but I
am reluctant to try to unpick his work. Where to go, please ?


texts: What are the next planned space telescopes?

texts: We've had hubble for two decades. Its become the most famous space telescope, and lately Kepler seems to be running for the second place. Are there any more powerful, better telescopes planned for launch in the near future?


texts: What are the main arguments against Hoag's object in particular and Hoag-like objects in general to be explained as Einstein rings formed by a farther edge-on galaxy?


texts: Is it possible to determine an observer's position on Earth from a photo of the night sky and the time of the shot?

texts:
To my knowledge, this question can be related to celestial navigation on one hand and to attitude (or orientation) determination from stars direction on the other hand. However the combination of this two methods does not solve the problem, so I am wondering if there is a different approach.

In celestial navigation, the altitude of the stars (elevation wrt to the horizon) at a given time can be used to determine the observer's position. The limit of this method is that the horizon must be visible, which is not necessarly the case in a photo of the night sky.

An image of the night sky can also provide information about the attitude (orientation) of the camera wrt to an inertial frame. The measured direction of a few stars (from their position on the image) and their known direction in an inertial frame give an estimate of the attitude. This method is used in devices called star trackers onboard satellites for instance.

The combination of this 2 approaches fails to solve the problem because the information about the horizon is still missing. Indeed, the orientation is found with respect to the inertial frame, not the local frame, and thus the horizon is still unknown.


texts: How to convert magnitude to spectral luminosity?

texts:
In the Gaia catalogue I have the magnitude of several stars in the G band. I also have the distances to these stars (parallax is known). Additionally, I oftentimes have the effective temperature in Kelvins.

Is there any way to convert this magnitude to an approximate spectral luminosity (units of W/Hz) at some frequency band? You may assume that we are in the Rayleigh Jean's regime (Intensity proportional to frequency2).


texts: What are the exact causes that differences in sunrises are shorter around june and december but long in march and september?

texts:
The changes of the sunrise and sunset times not expire regularly in a straight line but according to a sinusoid. Around the solstice (summer solstice on June 21st and winter solstice on December 21), the day length changes the least. The difference in sunrise and sunset in the days around the solstice is only a few tens of seconds per day.

At equinox (March 20 and September 23), the length of day and night on earth is everywhere the same. The difference in day length from day to day around the equinox is changing rapidly. The difference in the length of day can rise around the time of equinox to 3 minutes per day.

This asymmetrical lengthening and shortening of the days is because the "middle" of the day, every day a little shifts. This has to do with the fact that the earth does not describe a exact circle orbit around the sun, but an elliptical orbit. Because the Earth's axis at an angle to the orbit around the sun takes the shorter and longer are the days asymmetrical.

As for the elliptical orbit, I understand that, for example,the sun is the farthest from  the earth in the summer (on northern hemispere). Because of the (second) law of areas of Kepler is the speed of the earth there  the slowest. In itself, I understand then that at that time the difference in day length also is shorter. That is during June 21 (summer solstice).

But so is apparently also on the winter solstice on 21 December. Precisely at that time the earth is closest to the Sun (perihelion) so the earth has a higher speed. But because of the higher rate would you expect the day lengthening/shortening would therefore be greater. However, it appears not to be so. How is that possible?

So the question in short terms:

sunrise in june on the 21 at 5:30 and 22 at 5:31 etc
sunrise in march on the 21 at 7:30 and 22 at 7:33
So the difference is in june one minute and in march it is 3 minutes. And the question is what causes that the difference is bigger in march then in june. See http://www.timeanddate.com/sun/france/paris for a bigger disquisition


texts: Rotating observer plane inside celestial sphere with a given coordinates, time and date

texts:
I have plotted stars around the surface of a celestial sphere. At the centre of this sphere, I have a brown plane which is the land. This plane can be rotated in the x,y and z axis.

When x,y,z rotation of the plane is set to 0,0,0:

Front View:

Top View:

Bottom View:

After the user inputs the latitude, longitude, time and date, the plane should position itself correctly so that the correct stars are above the plane.

How do I accomplish this and what is the mathematics what links latitude, longitude and the rotation of Earth depending on the time and date to the rotation of the plane in 3-dimensions?


texts: Do you know about any Image Processing Challenges known in Space Telescope like Hubble?

texts:
For a computer science project  in University;

I need to find image processing problems on space exploration and Space Telescopes, only related to the image itself.

I have wrote a computer program that is able to separate every pixel of an image according to its pixel intensity (the whitest to the darkest) in 3 (or even more) big groups.

I was thinking that this might be somehow useful in a space image processing situation in a well known challenge of the technology in the Telescopes or machine vision devices around space.

Situation can be very specific, just as an example calculating the mass of a star by the look of the pixels which compose it, or something like this.


texts: Why is the moon's motion more complicated than that of those planets?

texts:
I am of course not an expert on astronomy. But from the literature, I have the impression that the lunar motion is very complicated. It seems that Kepler can make sense of the motion of the planets but cannot do that for the moon.

So, why is the moon's motion more complicated than that of planets like Mars?


texts:
In a star, the fact that not all the hydrogen is used up, not all the helium is used up etc. is a little counterintuitive.

Shouldn't it be that a star looses the hydrogen layer passively, in other words all the hydrogen eventually fuses into helium forming a helium star which then fuses all its helium into carbon forming a carbon star etc. all the way to iron? In this way you get up to the temperature required to form iron and nickel in stars with a much lower mass and in massive stars you might even have Iron being fused to form heavier metals.

Why isn't this so?


texts: Verifying refractor lens about scratches

texts:
I found an used Skywatcher 70mm telescope for buying, but its lenses are a little suspect about harming. In one photo, it looks clearly a scratch, but in the another, it is not visible. Could it be an optical illusion or is it really a harm? If it's a scratch, should I discard the possibility of buying?

Pointing where I think there are scratches:


texts: Why is there a supermassive black hole at the center of every galaxy?

texts: According to https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole) the gravitational sphere of influence of a supermassive black hole is really limited compared to the size of its hosting galaxy, why then is there one at the center of every galaxy?


texts: Photosphere is relatively transparent. Is that right?

texts:
This book, Astronomy in Minutes: 200 Key Concepts Explained in an Instant, by Giles Sparrow, says the photosphere is a visible surface at first, but after that, it says the Sun becomes cool and then transparent, so I am confused.

Our local star is a fiery ball of gas 1.39 million kilometres (863,000 miles) across. Powered by nuclear fusion, it has a three-layered internal structure (see pages 272 and 286). Its visible surface or 'photosphere' marks the zone where the Sun's hot gases grow sparse and cool enough to become transparent, at a temperature of around 5,500°C (9,900°F), but the Sun's sparse atmosphere or corona extends much further outwards.

I think the sentence highlighted in bold (not highlighted in the original) represents the definition of chromosphere... Is that right?


texts:
I am not very much interested in observational physics but I just like to look at the night sky. It gives me a lot of inspiration. So, i was thinking about making a telescope for basic observations. I googled about the design and found two models. One consisted of two convex lenses of different focal lengths and the other one consisted of a concave lens as the objective lens and a convex lens as the eye-piece(that's called a Galilean one I guess).

I am confused about which one should I make. Can anyone give me some insight into both these types?


texts: What would go into the design of a simple sky quality meter, used to measure night sky brightness?

texts:
@MCG's answer mentions several methods to classify night sky quality or brightness, and goes on to say:

  Additionally, you could always purchase a SQM (Sky quality meter) which is a small, portable battery powered device that you take out to your dark sky site, point it at the zenith, and the screen will display the darkness of the sky. Do beware that the SQM measures in 'magnitudes per square arcsecond' which means the higher the number, the darker the sky.

If I wanted to try to make a diy SQM, I'd need to understand if there was some established standards or practices associated with a sky quality measurement.

I might try to use a low dark current silicon photodiode in photoconductive or photovoltaic mode, low frequency couple it to a low noise op-amp, then do a cover-on measurement to zero out the leakage then do a over-off measurement to quantify the sky brightness.

What solid angle should it integrate over? Certainly more than one arcsecond, but would it be one square degree, or a 10 degree wide circle for example?
Silicon covers the visual spectrum and some near infrared as well. Depending on the protective materials over it, it might measure all the way out to a micron. Is that a problem, or is light pollution pretty much in the visible part of the spectrum (for obvious reasons) and I wouldn't need to hunt for an IR-blocking filter like the ones found on imagers for cameras.
With a single broadband (rather than spectrally resolved) measurement of photocurrent, I could choose a representative wavelength, or assume a flat 400-700 nm emission spectrum, or even make an educated guess about what the spectrum of noise pollution looks like, then use that along with an estimate of say 0.8 e-h pair per photon (roughly the shape of the sensitivity curve below) I could make some estimate on how much light was reaching the photodetector. But how might I convert that to magnitude? I understand the logarithmic scale, but right now I just start with 1300 W/m^2 is magnitude -27 (roughly the Sun's characteristic) as a reference. Is there anything better?
Anything else I haven't thought of?

above: Silicon photodetector typical spectral response, from here.


texts: Tools for creating a multiwavelength view of the sky

texts:
Not sure that this is the correct place to ask this question, but here goes.

I am trying to find tools to accumulate image data (preferably in FITS format) on a given (RA, Dec)-coordinate of sky. Right now there are several image services that offer cutouts, for example:

irsa.ipac.caltech.edu holds data from Spitzer, Planck, WMAP, IRAS, WISE
hla.stsci.edu holds Hubble Space Telescope data
skyserver.sdss.org/dr7/en/tools/chart/list.asp holds SDSS data
etc.

I wonder if anyone has written any code for accumulating these images given an argument of RA and Dec? Or if they can point me in the right direction of tools that they know exist? Any help would be appreciated.


texts:
What is the most accurate way of propagation of Earth trajectory relatively to the Sun? I need to calculate the Earth-Sun distance. Is there any library on Python?

I've used before JPL Horizons, however, it's not useful for propagation for 1 Myr.


texts:
Accretion discs are ubiquitous in astrophisics. As a direct corollary, they are important for the following question.

Consider the following model, representing one of the most simple models for accretion discs. A central object is a star (pre-MS, WD or NS, but not a BH) of mass $M$, surrounded by a thin flat disc of material, which continuosly feeds the star at a rate $\dot{M}$, such that $M/\dot{M}$ is much larger than thermal and dynamical timescale of the star (i.e. accretion rate is slow).

Everywhere in the accretion disc its local motion is nearly circular and nearly Keplerian. Therefore, at the interface of the star and the disc the disc will always tend to make the star rotate at nearly-Keplerian velocities. From the other hand, if the stellar outer parts were to rotate at nearly-Keplerian velocities, these parts would become gravitationaly detached from the star, which would have significant consequences for the stellar shape and structure. Surely, though, the process is going to be slow and the acquired angular momentum will be redistributed within the star.

Now the question: What will be happening to the star if it approaches nearly break-up velocities due to such a spin-up? This involves a few subquestions: How close the rotation rate can actually get to the critical one? If it can get close enough, how would the whole process look like? That is, what would happen in the short term to the star when the effects of rotation will start to affect its structure? What would happen to the star in the long term?

I would like to keep this problem as a purely hydrodynamical one. That is, assume, that the only laws involved are hydrodynamical and gravitational ones, with some constant accretion rate supported. In reality magnetic fields would also play an important role for some stars, and stellar winds could also possibly be important.

Examples of the decribed systems are numerous. It might concern cataclysmic variables, millisecond pulsars, pre-main sequence star in a protoplanetary disc, and many more.


texts: Graph of apparent magnitude of Mars through time from around 2000 to present?

texts:
I tried to search online but failed to find a graph of the apparent magnitude of Mars over time from 2000 to present.
From Wikipedia:

Mars made its closest approach to Earth and maximum apparent
brightness in nearly 60,000 years, 55,758,006 km (0.37271925 AU;
34,646,419 mi), magnitude −2.88, on 27 August 2003. ...
Every 15 to 17 years, Mars comes into opposition near its perihelion. These perihelic oppositions make a closer approach to
earth than other oppositions which occur every 2.1 years. Mars comes
into perihelic opposition in 2003, 2018 and 2035, with 2020 and 2033
being close to perihelic opposition.

So I am very curious how the present 2020 perihelic opposition apparent magnitude of Mars compare to the previous ones. It would be even more interesting if the graph also contains the distance to earth.
Could anyone point me to where I can find such a graph? Thanks.


texts: How do telescopes "zoom" and change angle of view?

texts:
Images from the Hubble Space Telescope have widely different angles of view. Is this only the result of composing multiple images, or can the telescope itself change its "zooming" optically, mechanically? How does the ability to change the angle of view differ between the major scientific telescopes of today and the near future, space and ground based?

And what about cameras on interplanetary probes? Can for example New Horizons optically change its zooming as it passes Pluto?


texts: Simple experimental evidence that Earth revolves around Sun

texts: What are the simplest experiments or calculations that give evidence that the earth revolves around the sun? Can you please explain them and reference the history? Many simple explanations such as this cite observations such as that relative position of two stars are observed from earth vary every night - which would not be true if the stars orbited the earth. But isn't the observation also consistent with a model where the stars orbit the earth but do so at different speeds, while the earth still orbits the sun? Simple explanations would be helpful.


texts: an exercise from Spherical Astronomy by Smart

texts:
I want to solve this exercise from Spherical Astronomy by Smart:

Prove that, in latitude 45°, the interval between the moment at which a star's azimuth is 90° east and the moment of setting is constant.

I can't understand it, it says prove that it is constant for a star, or it is constant for every star with different declinations?

Thanks.


texts: How far can we detect lightning in radioastronomy?

texts:
The wikipedia article on whistlers has this information:

Voyager 1 and 2 spacecraft detected whistler-like activity in the
vicinity of Jupiter known as "Jovian Whistlers", implying the presence
of lightning there.

This surprised me, because this implicates a short range for detection of lightning, and I thougth atmospheric discharges produced a lot of radio noise, and so radiotelescopes on Earth should have picked signs of jovian lightning before the voyagers. As it seems not to be the case, what makes lightining hard to detect at distance?


texts: What cluster of stars is this with a "dark donut" to one side?

texts:
In the 2011 Sixty Symbols video Spy Satellites (from Deep Sky Videos) after about 01:44 amateur astrophotographer Nik Szymanek shows an image of a cluster of stars and I think I see what looks like a dark donut shape (dust?) to the right of the center of brightness, as shown in the screenshots below.

The vertical streak is a satellite trail, the topic of the video but not related to the present question.

Question: Can someone recognize what object this is and explain what is thought to cause the dark donut?

Screenshots from video, click for full size:

Cropped and shamelessly processed to enhance the dark donut's visibility:


texts: Difference in redshift between 2 interacting galaxies

texts:
I have a galaxy 'A', say at redshift 1. Let's suppose this galaxy has no peculiar velocity. What would be the redshift of another galaxy 'B', that has a radial velocity of 500 km/s, relative to A?

All this in the standard (Planck) model.

Any tools to do get this easily (an python code, astropy?)


texts:
I wonder why saturn rings are so thin ? Rings A, B and C are approximately just 10 meters thick which is extremely thin compared to the size of Saturn. What are the explanations for that ?

Thanks a lot in advance for your answers.


texts: Is the expansion accelerating or Decelerating?

texts:
According to Hubble's Law, the farther a galaxy is, the farther it is moving away. But do we take into account the fact that we are actually looking in the past?

For example, there are two galaxies A and B at distance of 5 and 10 billion years respectively. Now, when we observe A we are looking at how it was moving 5 billion years ago. The same applies for B. So, now we conclude that 5 billion years ago space was expanding at a slower rate while it was expanding comparatively faster 10 billion years ago. What's wrong with this conclusion?


texts:
Are there any observatories with high powered telescopes that also allow public access?
Astronomy is a VERY expensive addiction, because no matter what telescope you get, you always want the next model up.   If we can find an observatory that allows the public to see through their scope in America or Canada, we can make our vacation plans accordingly.  Kids want to see Pluto.  Scopes that powerful are "slightly" over our budget.


texts: What is the current theory for the formation of the Earth's Moon?

texts: Given that the Earth and Moon are often said in the literature to have very similar geochemistry, what is the current theory as to how the Earth's Moon formed?


texts: If Mars orbited the Earth how distant would it have to be to cause the same tides?

texts:
If it were possible to replace the Moon with Mars, how distant would it have to be to essentially create the same oceanic tides as the Moon currently does? Mars seems to be roughly 3 times the mass of the Moon, so does that mean it should be 300% the distance?

Furthermore, would the increased distance cause any issues with Earth's orbit around the Sun or potentially cause some sort of situation like Charon and Pluto where the centre of mass is between them?


texts: Why aren't stars like Eternally Collapsing Objects?

texts: We know that stellar radiation pressure balances the gravitational compressive forced of a star. Are there other factors which resist such a collapse? Also, if radiation pressure balances gravitational compression, shouldn't the process of shrinking be a quasistatic process making stars Eternally Collapsing Objects (ECOs)?


texts: Slope of log plot of luminosity-velocity in Tulley Fisher relation

texts:
My partner and I did some computation on the Tully Fisher relation at uni.

In that, the slope of the logarithm plot between the luminosity and rotational velocity, was 2.6.

Now, that slope corresponds to the exponent to which $v_{rotation}$ is raised to. i.e.

$$L=Av_{rotation}^b$$

where $b$ is the exponent.

So, $b$ officially is close to 4. And I have used up a sample size of over 10 galaxies. And the plot looked pretty shabby honestly. Is our result poor?


texts: Shadows of Light = Space or Dark Matter?

texts:
Perhaps my title is misleading. but here is my understanding and I'm looking for confirmation.

You cannot have dark without light.

The only thing faster than light is darkness (i.e. shadows)

Stars are in the galaxy and are inherently bright.

Does this make "dark matter" mere shadows of extremely bright objects?

Is that also why it has to get dark before we can really see the majority of stars (subjective experience)?

Update:

With all due respect to user with answer below and thank you for taking the time to answer... I think this can be expanded on.

I have provided a source for my claims: youtu.be/JTvcpdfGUtQ

Check out the video, and it more explains my understanding of things...

Why I don't accept answer below:

Can you provide sources for your claims?

Also I didn't say you can't have DARK MATTER without Light , I said you can't have Dark, as in Darkness. I think you're getting hung up on Dark Matter which is a big part of my question, but not the entire question.

Also, are you claiming that all darkness in space is dark matter? I don't like to assume but for scientific purposes (not an accredited scientist) would we not agree that the measures made on dark matter were on a sample, and not the entire universe?

So to further question this, do shadows not exist in space?


texts: How many stars are in the constellation Canis Minor?

texts: I've read Wikipedia and found a list of stars in Canis Minor, and counted the number of stars in the list. I came up with 56 stars, a surprising number. I did some more research and found that Canis Minor was just the two binary stars orbiting together: Procyon and Gomeisa. So I came to the conclusion that the answer could be found here somewhere. Please help and tell me how many stars the constellation Canis Minor is made of. Thank you.


texts: What part of the EM spectrum was used in the black hole image?

texts: The EHT has published the first image of a black hole.  It is the event horizon of the singularity in M87 against its accretion disk.  I've been through the reports in the popular press but none mention the light wavelengths this was taken at.  Is it an image in visible light, infra-red or longer?


texts: Black holes have so much gravity that even light can't escape from them. If we can't see them, and the suck up all  electromagnetic radiation, then how can we find them?


texts: Is there any difference among exoplanets in composition due to host star?

texts: Simply put, would, say, rocky planets orbiting red dwarfs have a noticably different chemical makeup than rocky planets orbiting B-type main sequence stars? Mutatis mutandis gas giants and the other types of host stars.


texts: Why does east and west appear to be mirrored in the celestial sphere?

texts:
I'm wondering why East is left and West on the right of the celestial sphere if you look at images of it?
Example below:

(source)


texts:
In our solar system, MOIDs (minimum orbital intersection distance) of different planets reach a minimum of ~30 million miles (Mercury and Venus). However, other star systems have more compact planets. So I am wondering, what is the absolute minimum for planets of different sizes (list below)

1 Earth mass
5 Earth masses
10 Earth masses (Neptune-sized)
50 Earth masses (Super-Neptune)
100 Earth masses or 0.3 Jupiter mass (Sub-Jupiter)
1 Jupiter mass
5 Jupiter masses

P.S. Sorry if the list is too big, some can be omitted for sake of writing all of that.


texts:
Dark matter appears to dominate the matter component of the universe as compared to luminous, or baryonic, matter. Though it does not interact electromagnetically (it doesn't absorb, scatter, or emit photons), there is an ever-increasing mountain of evidence for its existence through it's gravitational interactions with stars, galaxies, and clusters, as well as it's influence on objects behind it through what's known as gravitational lensing.

My question is, what are the most promising particle candidates of dark matter, and which experiments currently exist (or will exist) to attempt to answer this question?


texts:
Not being a math-minded person at all, however basic it might appear to be to all of you in this particular question, I would like to get the following calculation confirmed.
I'm trying to calculate the angular diameter of two natural satellites orbiting a planet, as seen from the planet.

Satellite A is located 568350 km from the planet and has a diameter of 4400 km.
Satellite B is located 357094 km from the planet and has a diameter of 1900 km.

For Satellite A, we would therefore get:
$$\arctan\left(\frac{4400}{568350}\right)=0.44^\text{o}$$
For Satellite B, we would therefore get :
$$\arctan\left(\frac{1900}{357094}\right)=0.3^\text{o}$$
Similarly, for our moon, the same calculation goes as follows:
$$\arctan\left(\frac{3476}{384402}\right)=0.51^\text{o}$$


texts: Average number of exoplanets in a system

texts: I was just wondering what is the normal amount of planets is in a star system, because in my mind 8 planets seems like a relatively large amount. So are we above average? Can we even see all the exoplanets in a star system? Also, do we know if belts of asteroids are common? Is it even likely for a star to have planets?


texts: My Model 36050 telescope came with the H6mm, H20mm and the green filtered moon lenses. It has a 360mm focal length and a 50mm diamiter.  What can be seen with it?


texts:
I was just watching a lecture from Carl Sagan. He talked about figuring out the distance to the stars; it got me interested in learning more about the subject.

As far as I know, the Inverse square law and parallax can be used. Can anyone expand on these? Specifically with regards to what I could do to measure the distance from Earth to Proxima  Centauri.


texts: Did a piece of Halley's comet strike the Earth 1,500 years ago?

texts: A friend showed me an article saying that there's evidence that a piece of Halley's comet may have struck the earth about 1,500 years ago. Is anyone here aware of such evidence?


texts: Can we see asteroid 1998 OR2 with unaided eye?

texts:
Will I be able to see the asteroid 1998 OR2 with the naked eye on April 29?

If so, where can I find more information on when and where I'll be able to see it?


texts:
The moon follows 5.14 deg inclined path. the ecliptic and the Moon's path coincide are called 'nodes', and they travel around the ecliptic in a period of 18.66 years.

Once every 18.66 years, the ascending node coincides with the vernal equinox, and the Moon will reach declinations of about +28° and -28° (much more than the Sun, which reaches ±23°); this is called a 'major lunar standstill'. 9.33 years later, the opposite is true, and the Moon will only reach ±18° during a 'minor lunar standstill'.

I read https://answers.yahoo.com/question/index?qid=20121018091531AArHgNS&guccounter=1 here, the precise declination of moon is at +28.88 deg, over the earth. Where can I find the exact, value from other sources? I don't know how he found this value? "  I get plus or minus 28 degrees 53', over a 18.6 year cycle. Eventually it will cover the whole region, but it will take many thousands of years (depending on how close you consider it has to get to a point to cover it.) " 28 degrees and 53' = 28.88 deg

The 0.88 deg, to time is 53 min, I think it seems to coincide with the fact the moon rises, 53 min sooner, over each day. Is this related to this? 13.2 deg * 4 min/deg = 53 min

The maximum inclination of the moon, for standstill, can it be specified by the eclipse point? On, august 21 2017, an eclipse occurs in 28.88 deg on latitude point. Is this angle the same as the inclination angle specified here: http://www.umass.edu/sunwheel/pages/moonteaching.html - 28.8 deg.

I just want 28.88 deg = 28 deg 53' confirmed.


texts: "Extinct Species" of meteorite - how quantitative is the statistical analysis?

texts:
This recent paper in Nature A new type of solar-system material recovered from Ordovician marine limestone is available to read on line, and Figure 3 is already widely publicized.

  Comparison of Δ17O versus ε54Cr of achondrites, and ordinary chondrites fallen on Earth in recent time with Österplana 065 (Öst 65) and fossil L-chondritic meteorites Österplana 018, 029 and 032 (Öst 18, Öst 29 and Öst 32 in figure). The fields for ordinary chondrites (OC), Mars (SNCs), Earth and earth-likes, Vesta (HEDs), brachinites, ureilites, winonaites (win), acapulcoites (acp)/lodranites/ungrouped achondrites (ung) and a newly identified unique metal-rich chondrite are marked with select representative samples with available data. Symbol colours indicate meteorite type or grouping. Note that the carbonaceous chondrites and affiliated achondrites plot outside the field, with highly positive ε54Cr. The ε54Cr literature values are from refs 12, 18, 19, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, and Δ17O literature values are from refs 10, 12, 25, 30, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43.

The paper is long and careful, and in any scientific field of classification, there is a huge amount of work and thought and discussion that is sometimes difficult for those outside to understand without the necessary background.

The last two sentences of the abstract read:

  "...This may be the first documented example of an ‘extinct’ meteorite, that is, a meteorite type that does not fall on Earth today because its parent body has been consumed by collisions. The meteorites found on Earth today apparently do not give a full representation of the kind of bodies in the asteroid belt ~500 Myr ago."

In this part of the image, without knowing a lot about meteorite taxonomy but a little bit about error bars, the image alone doesn't convince me of much at all by itself. There are good mathematical techniques to try to find clustering in data, and probably some methodologies to apply to calculate the probability that the clustering is likely to arise by random chance or is likely to be "real" (which is just another way to say - albeit loosely - not likely to arise by random chance).

I'm guessing those colorized islands probably come from statistical analysis of a very large number of meteorites, but I don't know that for sure.

Are these color islands generally accepted and their shapes published independently somewhere? Is there a plot where I can see all of the points that were used to identify these clustering? How exactly were these contours drawn?

Finally, is there a statistical analysis using these error bars and contours that express the probability that the extinct meteorite specimen described in the paper is part of the distribution or not? Of course a number by itself is weak because of the softness of the assumptions, but is there one and if so how was it calculated?


texts: What is the last nuclear reaction in a binary system before supernova?

texts:
I understand that binary star systems may cause type Ia supernovae.

I have a question regarding supernova explosion in a binary system. What is the last nuclear reaction before massive explosion?

I am thinking of two 2 close stars of 3M and 6M.


texts: How are the ESPRESSO Echelle spectrograph's calibration "lines" produced?

texts:
The CBC News Technology and Science Q&A This ESPRESSO machine doesn't make coffee but scans the skies for habitable planets includes a nice description of ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) and links to the YouTube video Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations.

In the spectrogram shown at about 00:55 in the video I've noticed that each band is accompanied by a finely space array of points. I am assuming that they are some kind of calibration, but not sure how they are produced.

Since the orders are changing, I can't really figure out if these are equally spaced in wavelength or in frequency.

How are these densely space calibration "lines" produced?

You can see the full spectrogram in this question.

above: Cropped subsections from a HD Frame grab from the video at about 00:55. Recorded with Python using this script, then extracting frames using this script in Blender.


texts: Can a stellar mass black hole swallow Earth?

texts: In the Astronomy magazine I read that the stellar mass black holes are only the size of Rhode Island. Can they swallow Earth?


texts: Have we detected galaxies which have red-shifted beyond the visible light range?

texts:
According to this answer it is possible for galaxies' light to move beyond the visible frequencies due to redshift:

It is possible that eventually the light from them could move into the infrared and even
the microwave in extreme cases.

Could this have already happened?  Have we already looked for it?  Is there a telescope with the instrumentation to detect galaxies' 'light' at these wavelengths?


texts: Is there a theoretical limit on telescope's resolution?

texts:
Is there a theoretical limit on what we can see with a telescope of fixed size based on Earth's orbit or the Moon, that is in Earth's region of Solar system outside the Earth's atmosphere?

Why not instead of sending probes to Pluto and Ceres, just invest efforts into building better telescopes, so to photograph the surfaces of these bodies in greater detail? I suspect that all necessary information in fact reacheds Earth's region, it is just matter of sensibility and exposure to fix it, am I right?


texts: UK groups that investigate extraterrestrial life

texts: What are some UK based groups that research astrobiology, especially the chance of extraterrestrial intelligence?


texts: Gravitational field for oblate spheroid?

texts: Consider a planet, described as an oblate spheroid. Assume that the spheroid is uniformly dense but not a point source. Outside of the object, do all vectors in the gravity field point through a single point inside the planet?


texts: how to get image coordinates of source points in saoimage ds9?

texts: I loaded my fits image to saoimage ds9. Then, Analysis->Catalogs automatically brought up source locations.  how can i get xy image coordinates of this source locations?


texts: Why are nearby stars like Proxima Centauri and Barnard's star not visible to the naked eye?

texts: Proxima Centauri is the closest star to the earth other than the sun yet it isn't visible to the naked eye, but stars that are further away are easily found in the night sky. Why is that?


texts: Why does rotation prevent the further contraction of the cloud?

texts:
In the book Horizons: Exploring the Universe (2018) by M.Seeds and D.Backman, chapter 9, page 170-171, it states that:

At least four factors resist the
compression of an interstellar gas cloud, and those factors must
be overcome by gravity before star formation can begin. [...]
Third, everything in the Universe rotates. As a gas cloud begins
to contract, it spins more and more rapidly, just as ice-skaters
spin faster as they pull in their arms. This rotation can become
so rapid that it resists further contraction of the cloud.

Which bring the question of why does rotation prevent the further contraction of the cloud? Doesn't the rotation create a centripetal force that pointed inside, thus make the matters become denser, therefore it shouldn't be a reason that interstellar have to overcome in order to form stars?


texts:
A neighbor called me to ask about drones flying around. I managed to take several pictures and they don't look like any star, planet or other celestial body I have ever seen, but they seem too steady to be drones.

I managed to get one decent picture:

Any advice for figuring out whether that is a real celestial object or man-made?


texts: Can we actually predict an asteroid impact?

texts:
I have been seeing all these "hoaxes" on the internet about Planet X and how it would destroy Earth, on September 23rd 2017. It didn't happen.

I started to wonder if it's possible - with our current technology - to map the path of an asteroid with calculations of how an asteroid will pass near or collide with Earth. I've heard one is going to "safely" pass Earth on January 13th 2019.


texts:
It has been some time since I've seen the Moon. What are the conditions that might affect whether or not the Moon is visible within a 24-hour period?

Please take into consideration any notable astronomical effects: moon phases, light pollution, other viewing conditions. I want this to be applicable to other cases as well as my own.

Here is my particular case:

  I live in Wales. The date today is October 3, 2016. There has been no moon visible in the sky for the last 5 days, and I don't know why.


texts: Question from Introduction to Modern Cosmology by Andrew Liddle

texts:
The exact question goes like this:
In the real Universe the expansion is not completely uniform. Rather, galaxies exhibit
some random motion relative to the overall Hubble expansion, known as their
peculiar velocity and caused by the gravitational pull of their near neighbours. Supposing
that a typical (e.g. root mean square) galaxy peculiar velocity is 600 km s-1,
how far away would a galaxy have to be before it could be used to determine the
Hubble constant to ten per cent accuracy, supposing

(a) The true value of the Hubble constant is 100 km s-1 Mpc-1?

(b) The true value of the Hubble constant is 50 km s-1 Mpc-1

Assume in your calculation that the galaxy distance and redshift could be measured
exactly. Unfortunately, that is not true of real observations.

I started off by writing the hubbles equation and as they mentioned it should be accurate to 10% so its peculiar velocity (given as about 600 km/s) is less than or equal to 10% of its expansion velocity. But this gives me different answer.

The actual answer to the problem as stated in the book is
Slightly different answers are possible depending how you deal with the rms velocity.
You should get something like r > 35 Mpc for H0 = 100km s-1 and r > 70Mpc
for Ho = 50kms-1

What am I missing here? Has it got to do with the RMS velocity as mentioned? Please explain.


texts:
After watching this JPL video of a Cassini time lapse of Saturn, I wondered since being mostly gases, how do we know how long a day—one rotation period—is?

Is there a usable relation between how fast the upper most layer of gases is and the planet's solid core?


texts:
According to the big Bang Theory, the universe have started from a point and it describes how the universe expanded from a very high-density and high-temperature state.The initial singularity was a singularity of infinite density thought to have contained all of the mass and space-time of the Universe. If it was so dense,  why didn't it engulf the other parallel universes just like a supermassive black hole?

If there are no other universes,  why the existing black holes didn't suck the entire universe yet?


texts: From my understanding D4000 is a ratio between the 'flux densities between 4050 and 4250 Angstroms and that between 3750 and 3950 Angstrom', from Poggianti et al (1997). However I do not understand why this is a direct relationship with stellar age. Did someone just notice that D4000 has a strong correlation to stellar age, if so what is the scientific explanation of this relationship.


texts: What would it take to capture a passing asteroid or other celestial body into Earth's orbit?

texts: Every once in a while, we have a celestial body that passes through our solar system. What would be needed to reel one of these in? Not crashing down to Earth, that would be bad, but say into a high earth orbit (above the satellite belt)?


texts: Can a black hole with the density of water exist?

texts:
During an outreach event, the speaker mentioned the fact that black holes weren't necessarily dense objects and that black holes of any density could exist, even black holes with the density of water.

Now indeed, the schwarzschild radius of such a black hole would be
$R = c\sqrt{\frac{3}{8 \pi G \rho}}$
with $\rho$ the density of water.

But could such a non-dense black hole exist in practice? If not, what would prevent it from forming or from being stable?


texts: What should I look for in an astronomy tripod?

texts:
I have a Celstron SkyMaster 20x80 binoculars, and a tripod — Vanguard 263AT with a SBH-100 head.

The binoculars weighs 2.2kg, while the head is rated to a loading capacity of 10kg, and the legs to a tilted loading capacity of 5kg. This means that the tripod should work well, but it doesn't — it vibrates a lot even when my face gently touches the eyepiece, even without the center column extended, so much so that it becomes a pain to use the binoculars.

So I'm considering upgrading my tripod legs. I'd like this tripod to work well not just with the 20x80 binoculars I have, but also with a 25x100 pair of binoculars I'm considering buying (weight: 4.6kg), and maybe a small telescope (say with a 6-inch aperture).

Am I correct in understanding that:

I need to upgrade my tripod legs.
I don't need to upgrade the head, since it's rated to twice the capacity of the legs, and to twice the weight of the biggest binoculars I'll use.
I should look for legs that are rated to twice the weight of the biggest binoculars I'll use.
I should look for a set of wooden legs, since these have greater stability than aluminium legs while being affordable, unlike carbon fiber.
What else should I keep in mind when I look for an astronomy tripod?

I read that:

  [...] the most popular materials are aluminum, carbon fiber, and,
  somewhat surprisingly, wood. It’s one of those common compromises—pick
  two of the following three: weight, price, and/or vibration dampening.

  Wood is extremely good at absorbing vibrations, and is pretty
  affordable, but weighs a ton. Carbon fiber is light and stable, but
  you’ll pay for it. Aluminum is affordable, and fairly light, but is
  prone to channeling vibrations.

So I'll prioritise stability and affordability over weight.


texts: Telescopes for beginner astrophotography

texts:
I'm a fairly experienced photographer and have enjoyed taking lots of photos of the night sky with my existing gear - wide angle starscapes over pretty hills, mostly.

I'd like to move on, having seen what's possible with telescopes and mounted cameras. I've read around, and am now confused....

The plan sees me getting a telescope, tripod and mount of some sort (equatorial I believe I need?), and periodically putting that carefully into my car, driving into a rural area with dark skies and clear horizons, and pointing it at interesting things - my urban gardenless flat really isn't a good location for this.

I have Nikon full and crop frame and Olympus m4/3 cameras available - I believe I'd need a T mount adaptor to fit them? Is one likely to be hugely better than the other for this in a way I've not foreseen? I know the Nikon has much better low light performance but the Olympus puts more pixels on the subject with its small chip and has the very useful live time mode. I'm used to raw processing, but I'm expecting there'll be more specialist software I'll need to learn.

Am I anywhere near right or is this all mad?

What sort of things do I need to watch for on telescopes?
Am I going to trash a telescope by moving it around like this? I'm used to moving big lenses which I'm hoping is tolerably similar (accepting the very different optical designs)?
What have I completely missed?


texts: Cosmological redshift - How do we know it's not caused by the observer's time dilation?

texts:
Instead of cosmological redshift being caused by the metric expansion of space, why couldn't cosmological redshift be caused by our perspective moving faster in time?  Said another way, if the observer moved through time faster than the emission source, couldn't this time dilation be an explanation of cosmological redshift?  Instead of the metric expansion of space, why not "cosmic time dilation"?

As far as I know, there is no physical mechanism in physics to explain how the metric expansion can cause red shift.  It appears that we assume metric expansion can cause red shift with no underlying physics or testing.

Alternatively, red shift caused by time dilation is well tested, proven, and understood.  It is something we can play with here on Earth.

Wouldn't assuming cosmological redshift is caused by the observer moving faster in time than the emission source in the past makes more sense than the metric expansion of space causing cosmological redshift?

Of course when measuring redshift from distant sources, the Doppler and gravitational effects must be accounted for.  That's not what I'm asking about.

Edit:  I probably need some math help, or a pointer to a better example to play with.

With a Hubble Constant of 71.9, a lightyear expands about $7.3483\times10^{-11}\ \%$ per year.  The Pinwheel Galaxy is 20.87 million light years light years away.  If I continuously redshift the Pinwheel Galaxy by the expansion of space, I get a redshift of .00153 using this equation:

$z = 1 - e^{(7.3483\times10^{-11})(2.087\times10^{7})}\ $

The pinwheel galaxy has a measured Redshift of .000804.

Is there an equation that gives a better result without magic values for $z$ or a real world example with known values that works?


texts: If two comets come together in space, will they bounce apart, merge into a single body, or could they travel together through space, either touching or orbiting a shared center of gravity?


texts: What is the dimensions of the smallest object detectable by an optical fiber from a specific distance?

texts:
Optical fibers are well-known to observe astronomical objects, say, galaxies, to generate massive spectroscopic surveys. The galaxies are often very far from the optical fibers of a telescope. However, I'm thinking about the possibility of using optical fibers to observe much smaller objects (compared to galaxies) like satellites in much shorter ranges like from earth to GEO orbit.

In particular, given an object at the distance $r$ from an optical fiber mounted at the focal plane of a telescope, how big (in terms of dimensions) that object should be to be detectable by the signal coming from the optical fiber?  (I couldn't find any related specification in the web. Since I need to use the answer as a part of my research, any cited reference is extremely appreciated.)


texts: Question on dec 2013 answer to"How to measure the altitude and azimuth of a star?"

texts:
I am looking at your response to the above question.  You stated:

  For Northern Hemisphere, a star that is crossing the North local meridian (that is, between the Zenith and the North Horizon) the latitude is 90−alt=dec−lat

  For Northern Hemisphere, a star that is crossing the South local meridian (that is, between the Zenith and the South Horizon) the latitude is 90−alt=lat−dec.

I am simply not seeing this when I looked at image for the culimination of X in : http://star-www.st-and.ac.uk/~fv/webnotes/chapter4.htm" ???


texts: Are magnetic field lines Spiral for Spiral Galaxies? If no, then how it look like topologically?

texts:
I am an undergraduate doing summer project at USO. I am working on Sun's Magnetic field model. I got spiral graph for that, which looks like any spiral Galaxy. Do you think that there is some correlation between them. Or let me put this way, is magnetic field of Galaxy spiral?
Thank You in advance.


texts: Why do photos of Earth always have a black background?

texts:
if we are in the milky way, and people are able to take photographs from Earth that look like this:

Then how come almost all of Earth's photos look like this?

In the first photo, we are seeing what is "outside Earth", and so if we actually went "outside Earth", why don't we see these gases?

Is it because of the Sun blinding our vision? Suppose a satellite was hovering over the part of Earth where its night time (very little sun interference). If it "turned around and faced the opposite direction from earth", it would see the void, but would it see all these gases, or just the black void.

Why?


texts:
Saturn's rings will never clump together, because they are within the Roche limit. Which makes me wonder if a star could have rings that are kept from clumping together due to tidal forces. Have any ring systems been observed within the Roche limit of other stars?

The answers to this question consider rings in a more general sense. I would like to know specifically about rings around a star that exist due to the same mechanism as Saturn's rings.


texts: Would we have more than 8 minutes of light, if the sun "went out"?

texts:
The common theory is, that if the Sun "shut down", we would see the light for eight more minutes (the time that it takes the photons to reach the Earth).

However recently I have read that photons need around 100 000 years to reach the Earth, since the reactions are happening at Sun's core, and gamma rays can't leave the Sun without interacting with other particles, unlike neutrinos for example.

Is that theory correct? If the Sun's core "shut down", would we still receive photons (light) for another 100 000 years, with only neutrinos disappearing immediately?


texts: Please Help Me With Choosing my 1st Telescope

texts:
I'm going to buy my 1st telescope. With the budget, I've shortlisted some. Tried with the YT videos, reviews and guide. Few things still bugging me. Hope somebody can give me any advice on this. There are two very basic entry-level telescopes I've chosen. Please can somebody advise me which one is good.
Telescope 1
-Focal length:900mm
-Lens diameter: 60mm
-Optical system: Reflection
-Eyepiece:SR4mm/H12.5mm/H20mm
-Accuracy:1.842 second of arc
-Horizom:1.2 degree
-Zenital:90 degree
-Finderscope5*24 Optical Magnification
-Magnification:45/65/72/108/135/216/675X
Telescope 2
-Optical system: refraction
-Optical coating: multi-layer coating
-Objective lens diameter: 70mm
-Limit magnitude: 12
-Focal length: 300mm
-Focus ratio: 429
-Eyepiece: 6mm 20mm
Really appreciates your help.


texts: Is the Jupiter-Sun system considered a binary system of some type?

texts:
Since Jupiter is very massive, it is the only planet (in our solar system) that has a center of mass with the Sun that lies outside the volume of the Sun. (Source)

If Jupiter was a star, they would form a « binary star ».

If the Sun was a planet, they would form a « double planet ».

Since the Sun is a star and Jupiter is a planet, does this have a particular name?

Does Jupiter have a special status or a particular effect in our solar system because of its heavy mass?

Since Jupiter-Sun's center of mass lies outside the volume of the Sun, that means that the Sun moves around that center of mass. Does this have an effect on Mercury, Venus, Earth and Mars orbits?


texts: Has the most luminous object in the universe been found?

texts: It's hard to search through the entire (observable) universe, but the most luminous thing there is must on the other hand be easy to find. Can we now say that the most luminous quasar we know, is the (intrinsically) most luminous one that exists, or will we soon?


texts: What does black hole evaporation correspond to in the accelerating universe / black hole analogy?

texts:
In the same way, as a black hole emits Hawking radiation corresponding to its temperature

$$
T = \frac{\hbar}{r_{BH}}
$$

the accelerating universe can be described as all of the stuff falling towards the cosmic horizon, such that it is analogue to a black hole where the cosmic horizon takes the role of the event horizon with the black hole "interior" being outside.

The corresponding temperature of the accelerating universe is given by

$$
T = \hbar H = \frac{\hbar}{R_{H}}
$$

where H is the Hubble constant. Therefore, the cosmic horizon emits something similar to Hawking radiation too.

The fate of a black hole is to finally evaporate. My question is: What does black hole evaporation correspond to in the accelerating universe when making use of this analogy?


texts: How would one figure out the rate of axial precession of a planet?

texts:
I want to start with the fact that I am by no means an astronomer, or even a hobbyist in the field. I am attempting to build a fantasy world which still reflects accepted physical laws.

My problem comes from the fact that I have no idea how to go about figuring out the period of axial precession for a fantasy earth-like planet. For the Earth, I believe the period is roughly ~26,000 years. I've spent a few days trying to find resources, but in  all honesty I can't wrap my head around half of the equations I've seen presented.

How would I go about doing this calculation for a given planet, and which of its properties would I have to specify in order to do this calculation?

Edit Important? Details:
My Earthlike planet has a mass 1.2597x the mass of our earth, and two fairly large moons (actually, one of them may be large enough to classify it as a binary planet but I'm not sure).


texts: Is there a website out there where I can find/compare data on stellar parameters for the brightest stars?

texts:
I'm looking for some kind of catalog/repository which lists stellar parameters (e.g. mass, radius, temperature) in a similar vein to NASA's Exoplanet Catalog (but except for stars), for the brightest stars in the sky, with $m_v<4$. I'd rather not have to refer to Wikipedia as they only list data from one source (or an average of several uncited sources).
I'm aware of SIMBAD, but that site doesn't have a very intuitive UI and doesn't list mass measurements. I've also tried CDS (http://cdsportal.u-strasbg.fr/), but I can't find distance measurements from Gaia (although apparently Gaia doesn't measure distance and I need to put some kind of external study into a Gaia search query which I haven't got a clue how to use).


texts: How to calculate full (mechanical) energy on the hyperbolic orbit?

texts: For example, for a comet that travels on the hyperbolic orbit around the sun.


texts: How much do the mascons on the moon affect surface gravity?

texts: The gravity anomalies on the moon (generally accepted to be caused by mascons) are powerful enough to affect the orbit of satellites.  I'm trying to determine how much effect they would have on a person standing on the surface.  If we experience about 17% of Earth's gravity on the lunar surface, how much would that differ within one of these anomalies?


texts: How to convert a gravitational force to speed and direction

texts: I am doing some calculations, and I wanted to know that after using the gravity equation, $$F = G \frac{m_1m_2}{r^2}$$ if you can use $F$ and convert it into a speed, such as km/year, and also a direction or angle from the original direction. I am just looking into astronomy, and any help is appreciated.


texts: How to project an arbitrary elliptical orbit onto the night sky plane?

texts:
An ellipse E is arbitrarily oriented in 3-D space with the origin at one of its foci. Use the standard elements provided in the figure. Assume the reference direction $\gamma$ or $\gamma'$ is the positive Y-axis direction. Find the projected position vector of the ellipse onto the plane of reference as a function of the angle $\nu$.
After some calculation, I obtained the position vector of the celestial body to be
$$r=\frac{a(1−e^2)}{1+e\cos(\nu)}$$
Where $a$ is the semi-major axis and $e$ is the eccentricity. Now, I begin with the $(r,ν)$ coordinates in the plane of the ellipse and project it onto the reference plane using the $\cos(i)$ factor. How, then, can I find the orthogonal components $(x,y)$ of this projected position vector on the reference plane? A parameterized solution would be appreciated as well.


texts: Is it possible for the year to have 11 full moons (i.e. the opposite of a blue moon)

texts:
A blue moon is so called for when we have 13 full moons in a year.

https://en.wikipedia.org/wiki/Blue_moon

Is it possible to have 11 fun moons in a year? (Maybe if we included calendar changes, for example the start of the Gregorian calendar, this would be more likely)

If this is possible, what is this phenomenon called, and how rare are these years?


texts:
I see that there was already about Pluto's orbital inclination, but I would like to ask a more general question.

Why do planets have orbital inclinations?  And is there any particular reason, other than chance, for Mercury's inclination being so small?

Thank you!


texts: What does it mean for a black hole to "evaporate"?

texts:
It says in this Wikipedia's article:

  Entropy, however, implies heat and therefore temperature. The loss of energy also suggests that black holes do not last forever, but rather "evaporate" slowly.

Would a black hole, given enough time, dissapear? Has this happened before?


texts: How does the specific angular momentum - mass relation arise?

texts: I'm reading about the specific angular momentum - mass relation of galaxies, and I'm confident I understand what the possible reasons are for the difference in specific angular momentum of ellipticals vs. spiral galaxies. But I'm not sure how the relation itself arises. Any help would be appreciated, thanks!


texts: Is there a star simulation software that can handle mass ejections and supernovae?

texts:
I use MESA right now to play around with stellar dynamics, but it can't do mass ejections and therefore, I can't think of a way to let the simulated stars explode as supernovae.

What simulation software would I use for these?


texts: If Saturn's rings cannot coalesce into a moon because of tidal forces, then how are shepherd moons able to exist?

texts:
From Wikipedia:

In celestial mechanics, the Roche limit, also called Roche radius, is the distance within which a celestial body, held together only by its own force of gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.
Inside the Roche limit, orbiting material disperses and forms rings, whereas outside the limit material tends to coalesce.

However if tidal forces are strong enough to prevent the rings from coalescing, then how are shepherd moons such as Pan and Daphnis able to exist without being disintegrated by Saturn's tidal forces despite literally orbiting within the rings themselves?


texts: How are the gaps filled in images captured by CCD arrays?

texts:
Large telescopes don't use a single CCD; they use arrays of them. Since CCDs can't be seamlessly joined, this means that every captured image must have gaps. But, the resulting images almost never have gaps when they are presented to the public.

How are these gaps filled? Does the telescope capture a number of overlapping images and then merge the results? Wouldn't this result in differing exposure levels for differing areas of the image, which, even though it could be compensated for in the processing, would give different noise characteristics to the different areas?


texts: Is this image captured by the Hubble Telescope an original image?

texts: Is this image an original picture which is not photoshoped? I see a huge number of Hubble Telescope images on Google and I'm curious whether they are real images or not. If not, where I can find the real images captured by the Telescope which are not photoshopped?


texts: Assuming a hypothetical system without gas but only solid rocks, how big of a planet can form through the coalescing of these rocks?

texts:
I have heard that gas giants are primarily huge solid bodies like regular rocky planets that exponentially gained more and more gas in their atmosphere through their increase in mass which  they use to  pick up even more mass ,gaining even more gas, and practically any planet we know today above a certain size threshold is a gas giant
That being said, assuming an area in space with very little or no gas but a bountiful supply of rocks, how big can a solid planet consisting of these coalesced rocks get?


texts: Is it possible to use the vacuum of space to help with pollution

texts: Would it be possible to use a pipeline from the ground and uses the vacuum of space to dispose of things that are hazardous to the environment? (Liquid waste in particular comes to mind) Essentially acting as a syphon that shoots the waste off into the universe?


texts: Are sunspots named or individually tracked?

texts:
This is a follow-up question to How are the solar flares from May 23, 2021 extraordinary? which is about news of spaceweather.org. In the present question, I am curious solely about sun-spots and why some of them seem to have names like AR2824, and how they are tracked.
So what exactly is that name? Can I find a history about each sun-spot? How are sunspots tracked?
References

Is there a database of past sunspots?


texts:
Astronomical units of measurement are mostly pretty straight-forward:

Astronomical Units refer to the mean Earth-Sun distance (~150 million km or 93 million miles)
Light years are the distance light travels in a year (~9.46 × 10^12 km)

Another astronomical unit of measurement is the parsec.  What is a parsec and how is it measured?


texts: Do we know if we are overtaking or falling behind our arm?

texts:
Here's our galaxy as best we know, courtesy of this Wikipedia article:

It seems that as best we know, each arm is a compressed or brighter area (rather like a wave).

Looking at that image, do we know/guess if the arm-patterns are rotating? Or do they just sit there statically, unchanging from that diagram? Which way does it rotate, cw or acw per that diagram? (If we know.)
If the pattern is rotating - how long does it take it to rotate 360 degrees? As far as we know?

And hence ..

Indeed, is the arm-pattern catching us and moving through us ... or is the arm pattern slower than us and we're moving through it?


texts:
How to navigate with possible maximal precision using moon phases?

The Moon is the brightest celestial body seen in the night sky, and it is possible to find even through moderate clouds. So it's a good natural object to use in navigation when a compass is not available.

A full Moon is quite an easy case - it's directly on the opposite site from the Sun. However, in the other phases, are there tables or mnemotechnical methods allowing to determine the north based on moon shape, assuming the time is known? Are they different depending on geographical height? Is it possible to estimate both north and the time of the night, when only the geographical height is known?